When supplying electric power to a resistive load having parasitic inductance, a pulse width modulation (PWM) control is used that switches output voltage by means of a semiconductor switch. When the semiconductor switch in such a switching output circuit switches from a conduction state to a non-conduction state, surge voltage arises that is caused by release of electromagnetic energy accumulated in the parasitic inductance. The surge voltage causes a breakdown of the semiconductor switch.
In order to prevent the breakdown of the semiconductor switch due to the surge voltage, a snubber circuit is often used that absorbs the electromagnetic energy accumulated in the parasitic inductance of the resistive load. A switching output circuit in which a snubber circuit is placed is described in Patent Literature 1, for example. FIG. 16 illustrates a circuit block diagram of the switching output circuit by Patent Literature 1.
In a switching output circuit 900 in FIG. 16, when semiconductor switches SW1 to SW4 in a switching unit 910 are in the off position, a capacitor C is charged up to the voltage of a direct-current power supply 930 at the polarity that its terminal connected to a smoothing coil 940 has positive polarity. When semiconductor switches SW1 and SW3 turn ON under this condition, a current flows in a path of capacitor C→semiconductor switch SW1→load 920→semiconductor switch SW3→capacitor C, so that the charge in the capacitor C is discharged to the load 920.
When the charge in the capacitor C is discharged and the voltage becomes zero, the diodes in the semiconductor switches SW2 and SW4 turn ON automatically. This makes the current keep flowing circulating through all the semiconductor switches SW1 to SW4. The current flowing through the load 920 attenuates due to an equivalent resistance R of the load 920.
Next, when all the semiconductor switches SW1 to SW4 are turned off, the capacitor C is charged with the current flowing through the load 920 via the diodes in the semiconductor switches SW2 and SW4. The voltage of the capacitor C increases until the current comes to a stop. The electromagnetic energy has transferred from the load 920 to the capacitor C at the time of the stop of the current. At this time, the terminal of the capacitor C that is connected to the smoothing coil 940 has positive polarity, which is the same all the time regardless of the direction of the current flowing through the load 920.
Because the resonant frequency by the electrostatic capacitance C of the capacitor C and the inductance L of the load 920 is made higher than the frequency of the alternating pulsed current that arises, the semiconductor switches SW1 to SW4 perform a zero-voltage-switching and a zero-current-switching. That is to say, the circuit is configured to regenerate the electromagnetic energy of the load 920 and generate an alternating pulsed current in the load 920.